The inclinometer (tilt meter) has a wide range of applications, such as position determination in construction engineering, level measurement for mechanical platforms, monitoring of balancing system for automotive and aircrafts, monitoring in inclination and deformation of bridges and railroads, generating auxiliary horizontal lines for viewfinders of camera, tilt control and applications of handsets etc. It is also useful in the technical fields of semiconductor, chemical and biomedical engineering. The conventional micro inclinometers is divided into three types, according to their method of detection. They are: The mechanical inclinometer, the gas inclinometer and the liquid capacitive inclinometer.
The mechanical inclinometer comprises a mass block. The mass has two electrodes at its two ends, which interacts with two corresponding fixed electrodes under the influence of the gravity force, when the inclinometer tilts. The capacitance created by each pair of the fixed electrode and the mass electrode varies in response to the tilt angle of the inclinometer and is used to measure the tile angle of the inclinometer. The mechanical inclinometer is easy to fabricate but the spring that supports the mass block is fragile and tends to break when external forces are applied.
The gas inclinometer provides a sealed chamber, in which reference gas is enclosed. Heaters are used to heat the reference gas, whereby when the inclinometer tilts, the thermal convection in the sealed chamber varies. Variations in resistance at the thermistors adjacent to the heaters are used to calculate the tilt angle of the inclinometer. The gas inclinometer has a relatively simple structure and the reliability is not easily impacted by the dimensional variation of the microstructure. It, however, has drawbacks in that an additional sealing process of the chamber is required and that its reaction speed is relatively slow.
The conventional liquid capacitive micro inclinometer has a sealed chamber, in which electrolyte liquid is filled. The electrolyte liquid is conductive. Two electrodes partially immersed in the electrolyte liquid have substantially the same resistance, when the chamber is not tile. When the chamber inclines, areas of the electrodes immersed in the liquid vary relatively, such that differences in resistance of the two electrodes are generated. A reading circuit converts the variation of the tilt angle into electrical signals, whereby the tilt angle is known. The liquid inclinometer is simple in structure and fast in reaction but in fabricating the inclinometer, an additional sealing step of the chamber is required.
Most inclinometers are fabricated in two parts. The sensor part is fabricated in an MEMS (microelectromechanical system) process and the reading circuit is fabricated in a CMOS (complementary metal-oxide semiconductor) process. The two-part fabrication process is not only costly but also makes further condensation of the inclinometer difficult or impossible. A single-step MEMS process enables the design flexibility of the microstructure. Yet there is no standardized MEMS process that provides both flexibility in design and compatibility with necessary circuits at the same time.
Taiwan Patent No. 522221 discloses an inclinometer that comprises a printed circuit board and a pair of differential electrodes electrically independent from each other. The pair of differential electrodes and a common electrodes are enclosed in a sealed chamber and the sealed chamber is filled with dielectric liquid. When the inclinometer tilts, areas of differential electrodes that are immersed in the dielectric liquid vary, resulted in variations in the capacitance generated by each differential electrode and the common electrode. The capacitance of each differential electrode is measured to calculate the tilt angle. Such inclinometer is not fabricated in the MEMS process, therefore is bulky.
Japan published patent application JP 2008-261695 discloses a micro inclinometer that uses the same theory of the TW 522221 and has a structure similar to that of the TW 522221, while the liquid filled in the chamber is a conductive liquid. The micro inclinometer is fabricated using the MEMS process, therefore has a microstructure. However, the structure disclosed in JP 2008-261695 is not suitable for the standard CMOS process. Therefore, its production cost is relatively high. In addition, its differential electrodes are semicircular in shape, whereby its sensing accuracy is limited; the inclinometer so prepared is not for sophisticate applications. Nevertheless, the sensor and the reading circuit are prepared separately, making their integration difficult.